I was listening to NPR this morning and they were reporting on the delay of the launch of the Space Shuttle due to a faulty fuel sensor. This was while waking up so having been half asleep I may have heard this wrong but they seemed to be saying the problem with a faulty fuel sensor would be if the Shuttle ran out of fuel while the engines were running. The fuel sensors would signal to shut down the engines before this happened.
Seems to me just running out of gas (I know it is not “gas” in the shuttle) would be the real problem but then IANARocketScientist.
I think the problem is that the bad sensor could think the tank was out of fuel and shut down the engines prematurely. I assume they are designed to shut down the engines when the tank is empty for a reason, probably to have a proper shut down rather than one based on sputtering out of fuel.
The liquid hydrogen and oxygen in the large external tank runs the Space Shuttle main engines (the three large engines at the tail of the Shuttle). IIRC, the fuel lines are routed so that the fuel cools the engine before it reaches the combustion chamber.
If they were to run the tank completely dry, there would be a few seconds where the lines encircling the engine had no fuel in them (and therefore not provide any cooloing ability), but the engines would still be firing and producing a lot of heat. They intend to re-use the engines on the next flight, and running like that would damage them.
Damage which could include the engines disintegrating and spewing schrapnel around the shuttle - not a good thing.
NASA engineers are not sure a this point whether the probelem is is the external wiring/circuitry for the sensor (which could possibly be fixed in time for a Saturday launch, more likely Monday) or in the sensor itself, which is inside the fuel tank and much more difficult to repair/replace. If it’s the sensor, NASA says it could not be fixed before the July launch window closes. The next launch window is in September.
The liquid hydrogen goes through a fairly complex path on the way to the combustion chamber. First its pressure is boosted by a low-pressure turbopump, then a high-pressure turbopump running at very high power levels. Then the fuel is routed through many small pipes and manifolds which encircle the engine bell, combustion chamber, hot gas paths and gas generators. The cold hydrogen cools these hot parts, keeping them from melting. The now not so cold hydrogen gas is fed into the gas generators, where it is burned with some of the liquid oxygen (which comes from its own set of turbopumps) to form hot, hydrogen-rich steam. This hot gas turns turbines to power the hydrogen and oxygen high-pressure turbopumps, then is fed into the main combustion chamber and burned with the rest of the liquid oxygen.
Running the engine dry on hydrogen is bad. First, you’ll lose your source of cooling before combustion stops. Secondly, as the combustion in the gas generators goes from hydrogen-rich to a oxygen-rich as the hydrogen drops off, the temperature in the generators will increase significantly. With oxygen-rich combustion and no cooling, the walls of the hot gas paths and combustion chamber will burn through. The combination of hotter gas coming out of the gas generators and no hydrogen coming into the high-pressure hydrogen turbopump can cause the turbine to overspeed and disintergrate.
Basically, you’ll at the very least ruin the engines, and may cause an explosion that damages other things in the engine compartment.
On a normal mission the fuel cutoff sensors aren’t needed - the shuttle shuts the engines off when a predetermined velocity is reached, before the fuel tank runs dry. But certain abort modes require the engines to run as long as possible, and a fuel leak in an engine can cause the tank to run dry prematurely. IIRC, this has actually happened once - a small leak in an engine cooling line caused a slightly premature shutdown on one mission, although the shuttle was able to make it to orbit anyway.